Many communications systems in use today use a combination of electrical and optical components to transmit data. Due to its high bandwidth and other advantages optical systems are often preferred for many communications applications over purely electrical communications systems. Unfortunately, due to the limitations of existing optical components, it is sometimes necessary to convert optical signals into electrical signals, perform a switching operation on the electrical signals, and then convert the switched electrical signals back into optical signals for further transmission. The need to perform electrical switching operations is particularly prevalent in the case of time division multiplexed systems due to the limitations of known optical switches to perform time slot switching in the optical domain.
Communication techniques such as TDM and WDM allow several channels of data to travel over a single communications line. In the case of optical time division multiplexing (OTDM) and optical wavelength division multiplexing (OWDM) the communications line is normally an optical fiber.
In OTDM different channels correspond to different time slots, with the data on a communications line corresponding to different channels being interspaced according to time slot allocation. In OWDM each channel corresponds to a different wavelength with the potential for multiple wavelengths, and thus channels, to be transmitted in parallel using a single communications line. In some communications systems a combination of the two methods is used to transmit data.
Adding, dropping and switching channels are basic functions needed to manage an optical transport network (OTN). In WDM, a channel is represented by a single wavelength and these functions can be performed, as is known in the art, by using an optical cross-connect (OXC) and/or an optical add/drop multiplexer (OADM) that is wavelength selective. Such devices can be implemented using micro electrical mechanical systems (MEMS), waveguides and/or liquid crystal technology.
FIG. 1 illustrates an exemplary known 2×2 wavelength switch 100. The switch 100 includes two inputs and two outputs. Each input and output corresponds to a different line, e.g., fiber optic line, 104, 106, 108, 110 which is used to carry multiple wavelengths of light. Each wavelength λ corresponds to a different communications channel. In FIG. 1, input line 104 carries, λ1–λ4 corresponding to a first set of four channels while input line 106 carries λ5–λ8 corresponding to another four input channels.
The switch 100 includes first and second wavelength division demultiplexers 103, 105, an optical cross connect switch (OXC) 102 and first and second wavelength division output multiplexers 107, 109.
First and second input multiplexers couple the first and second inputs of switch 100 to inputs of the optical cross connect switch (OXC) 102. The demultiplexers 103, 105 divide the inputs into individual channels according to wavelength. Each wavelength output by a demultiplexer 103, 105 is supplied to a different input of the OXC 102. The OXC 102 is capable of directing the signal received on any input to any output of the OXC 102. In this manner, the OXC 102 can be used to redirect input signals corresponding to line 104, 106 to the inputs of either of the first and second multiplexers 107, 109, corresponding to output lines 108, 110, respectively. Output multiplexers 107, 108 combine the wavelengths received at their inputs and output them on a single line 108, 110, respectively. By using the ability of the OXC 102 to redirect input signals to any output, it is possible to perform wavelength switching as shown in FIG. 1.
In the example of FIG. 1, channels λ6 and λ7 which are received on the second input line are switched so that they share the first output line 108 with channels λ1 and λ4 which were received on the first input line 104. In addition, channels λ2 and λ3 which are received on the first input line 104 are switched so that they share the second output line 110 with channels λ1 and λ8 which were received on the second input line 106.
Adding, dropping and switching channels has proven more difficult to implement in OTDM systems than OWDM systems since the same wavelength is used for multiple channels making it more difficult to separate out the different channels for optical signal processing. The difficulty of performing time slot switching in the optical domain has caused system designers to resort to performing time slot switching in the electrical, as opposed to optical, domain. Normally, in the case of OTDM, optical-electrical-optical (OEO) conversion is performed with the channels being manipulated, e.g., switched, in the electrical domain. The O-E-O conversions are bit-rate and wavelength dependent and often require complex circuits. A discussion of optical to electrical conversion for an OTDM application can be found in U.S. Pat. No. 5,278,698.
The OEO conversion has the potential to become a bottleneck as transfer rates increase. Optical processing has the potential for avoiding such a bottleneck while offering the potential for an end to end optical communications system.
Advances in communications technology are allowing communications, e.g., OTDM, networks to reach data transfers speeds of up to 160 Gbit/s, and beyond. Accordingly, there is an increasing need for new methods and apparatus for performing channel drop, add and switching functions in OTDM systems. As discussed above, it is desirable that any new methods and apparatus for performing such functions be capable of performing the functions in the optical domain, thereby eliminating the need to perform OEO conversion. It is also desirable that any new methods be capable of being combined with OWDM techniques to provide, at least in some embodiments, a device capable of performing both OTDM and OWDM functions. Such a device would then be suitable for use in hybrid networks that use a combination of multiplexing techniques, e.g., WDM and OTDM techniques.
The absence of OEO conversion has the potential of lowering the cost and complexity of an OTDM switching device, while allowing the device to process data more efficiently thereby allowing for potentially higher throughputs than the known OEO techniques.
In view of the above discussion, it is apparent that there is a need for improved methods and apparatus for performing adding, dropping and/or channel switching operations in systems which use OTDM.